BACKGROUND OF THE INVENTION Field of The Invention

The subject invention relates to a controller for supplying a switching signal to a switched mode power supply.

Description of The Related Art Switched mode power supplies are extensively used in applications where efficient and compact power conversion is required. While there are many topologies and implementations of switched mode power supplies, all variations have square- wave signals for driving the power switches. These square-wave signals are normally generated by a pulse-width modulation controller and amplified by a buffer/driver which interfaces with the gate/base of the power switches. Low complexity, ease of implementation, well-understood operation and commercial availability of components have made square-wave control the de- facto standard in switched mode power supply applications.

It has been identified that the square-wave control is a major contributor of radiated EMI noise in the television receiver power supply. This can be explained in terms of sharp di/dt and dv/dt transitions caused by the square- wave control. The parasitic elements in the drive circuit are excited during transitions and create high frequency ringings which compound the EMI problem. The net effect is that the contribution of the control circuit to the radiated EMI signals is significant. This can result in noise visible on the display screen of the television receiver during reception of low level signals. In order to minimize this noise, most other approaches use power stage elements (such as snubber, resonant techniques, etc.) to try and reduce the noise. However, they do not have any impact on the drive circuit transitions which contribute significantly to the noise generation. The only other known possible approach to reduce the noise contribution is to introduce RDC damping circuits (also know as gate slow-down approaches) in the drive path. While these circuits have low complexity, they have limited effectiveness due to two reasons. First, they still do not eliminate fast transitions (sharp edges) in the control circuit. Second, they add significant dissipation in the power device when the

2 damping level is increased.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a controller for a switched mode power supply which minimizes the amount of EMI generated.

It is a further object of the subject invention to provide a controller for a switched mode power supply which is more efficient at reducing EMI than prior art controllers.

To this end, a first aspect of the invention provides a controller as defined in claim 1. A second aspect provides a method as defined in claim 5. Advantageous embodiments are defined in the dependent claims.

The above objects are achieved in a controller for supplying a switching signal to a switched mode power supply, characterized in that said controller comprises means for generating a sinusoidal-like wave, and means for selectively shifting a dc level in said sinusoidal-like wave whereby a duty cycle of said sinusoidal-like wave at a predetermined threshold level, corresponding to a turn-on level of a power switch in said switched mode power supply, is effectively controlled.

Applicants have found that if the square-wave controller is replaced by a sinusoidal controller operating at the same switching frequency, the di/dt and dv/dt transitions can be minimized.

Due to the lack of the square-wave waveforms in the controller, parasitic ringings are absent in the circuit. Additionally, a sinusoidal-like waveform applied to the gate of a MOSFET acting as the power switch in the switched mode power supply, results in slow turn-on and turn-off of the device, which further reduces parasitic ringings associated with the switching. It can be shown that with sinusoidal-like waveforms, the controlled slow-down can be optimally achieved without excessive power dissipation.

BRIEF DESCRIPTION OF THE DRAWINGS With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram of a switched mode power supply with a conventional square-wave controller;

Fig. 2 is a simplified block diagram of the sinusoidal pulse width

modulated drive controller of the subject invention;

Fig. 3 is a simplified block diagram of the controller of Fig. 2 with an additional stand-by control circuit; and

Fig. 4 is a practical implementation of the controller of Fig. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a switched mode power supply of the flyback type widely used in television receiver power supply applications, and having a conventional square-wave controller. An input ac signal is applied to an input filter/rectifier circuit 10 having a connection to ground and an output coupled to one end of the primary winding 12 of a transformer Tl. The other end of the primary winding 12 is connected through the source/drain junction of MOSFET Ql to ground.

The transformer Tl further includes a first secondary winding 14 having the series arrangement of a first output diode DR1 and a first capacitor Cf 1 connecting the ends of the first secondary winding 14. A first output voltage VO1 is taken from the junction of the diode DR1 and the capacitor Cfl while the opposite end of the capacitor Cfl is connected to ground. Similarly, the transformer Tl is shown having a second secondary winding 16 with a similarly arranged second diode DR2 and second capacitor Cf2, and generating a second output voltage VO2. While not shown in the drawing, the first output voltage VO1 is regulated and is used for generating a control signal for a controller. In particular, the first output voltage VO1 is applied to an opto-driver and controller 18 which applies an output signal FBO to an opto-isolator 20. The output signal from the opto-isolator 20 is applied as a feedback signal to the square-wave controller 22. The square-wave controller 22 includes an error amplifier 24 for receiving the feedback signal FBO from the opto-isolator 20. An output from the error amplifier 24 is applied to a first input of a PWM comparator 26. A second input of the PWM comparator 26 receives the output signal from a ramp signal generator 28. The output from the PWM comparator 26 is applied to a buffer/driver circuit 30, the output of this buffer/driver circuit 30 forming the output of the square-wave controller 22 which is applied to a gate of the MOSFET Ql.

Fig. 2 shows a simplified block diagram of the controller of the subject invention. In particular, as with the square-wave controller 22, the output from the opto- isolator 20 is applied to an error amplifier 32 which forms a corresponding dc level shift

DCLS signal. The output from the error amplifier 32 is applied to one input of an adder circuit 34. The other input of the adder circuit 34 receives a sinusoidal-like waveform from a sine-wave oscillator 36. The output from the adder circuit 34 is applied to a power amplifier 38 so as to provide the necessary drive capabilities. The output from this power amplifier 38, constituting the output of the controller, is applied to the gate of the MOSFET Tl.

The above description provides a controller for achieving a sinusoidal-like drive signal for pulse width modulation control. One direct extension of the above is a controller which generates drive waveforms for more than one switch operating at the same frequency. In such a case, the same oscillator may be used with different adder circuits to create the different drive waveforms. As a result, the invention may make use of a stand-by controller in addition to the main controller as shown in Fig. 3. In particular, a second output from the sine-wave oscillator 36 is applied to a stand-by adder 40 which also receives the output from a stand-by error amplifier 42. The stand-by error amplifier 42 receives as an input the stand-by sensed voltage SSV from the stand-by power supply (not shown). The output from the stand-by adder 40 is then used to control the stand-by power supply in the same manner as the output from the adder 34 and power supply 38 controls the switched mode power supply.

Fig. 4 shows a practical embodiment of the controller of the subject invention.

The controller 22 comprises an error amplifier 32, an adder circuit 34, a sine-wave oscillator 36 and a power amplifier 38.

The sine-wave oscillator 36 comprises a first operational amplifier OA1 commercially available as LM741, a second operational amplifier OA2 commercially available as LM741, and a voltage controlled oscillator IC1 commercially available as

ICL8038 from Harris for generating a sine-wave. The first operational amplifier OA1 has a second terminal 2 being coupled to a fourth terminal of the oscillator IC1 via a resistor R2, a third terminal 3 being coupled to ground via a controllable resistor Rl, a fourth terminal 4 being coupled to a common line, a sixth terminal 6 being coupled to an eighth terminal 8 of the oscillator IC1 via a resistor R4, and a seventh terminal 7 being coupled to a supply voltage + 15V. A controllable resistor R3 is coupled between a first 1 and a fifth 5 terminal of the first operational amplifier OA1. A slider terminal of the resistor R3 is coupled to the common line. A capacitor Cl is coupled between the second 2 and the sixth 6 terminal of the first operational amplifier OA1. Not mentioned terminals are not connected. The oscillator

IC1 has a first terminal 1 being coupled to a slider terminal of a controllable resistor R14, a fifth terminal 5 being coupled to a junction of a resistor R6 and a resistor R8, a sixth terminal 6 being coupled to ground, a ninth terminal 9 being coupled to ground via a resistor R9, a tenth terminal 10 being coupled to the common line via a capacitor C2, an eleventh terminal 11 being coupled to the common line, a second terminal 2 being coupled to a capacitor C3, and a twelfth terminal 12 being coupled to a slider terminal of a controllable resistor R12. The eighth terminal 8 is further coupled to the ground via a zener diode Dl. The remaining terminals of the oscillator IC1 are not connected. The free end of the resistor R6 is coupled to the fourth terminal 4 of the integrated circuit IC1 via a series arrangement of a resistor R5 and a resistor R7. A free end of the resistor R8 is connected to a slider terminal of a controllable resistor RIO. The remaining ends of the resistor RIO are connected to ground and to the common line, respectively. The resistor R12 further has a first free end being connected to ground via a resistor Rl 1 and a second free end being connected to the common line. The resistor R14 further has a first free end being connected to the common line via a resistor R13 and a second free end being connected to the ground.

The second operational amplifier OA2 has a second 2 and a sixth 6 terminal being interconnected, a third terminal 3 being coupled to a junction of a free end of the capacitor C3 and a resistor R15, a fourth terminal 4 being coupled to the common line, and a seventh terminal 7 being coupled to ground. A free end of the resistor R15 is coupled to ground.

The error amplifier 32 comprises a third operational amplifier OA3 commercially available as LM358. The third operational amplifier OA3 has a sixth terminal 6 being coupled to receive the feedback signal FBO via a resistor R32, a fifth terminal 5 being coupled to ground via a zener diode D3 and to the supply voltage + 15V via a resistor R34, and a third 3 and a fourth 4 terminal both being coupled to a junction of a zener diode D2 and a resistor R200. An other end of the zener diode D2 being an anode is coupled to ground, and a free end of the resistor R200 is coupled to the supply voltage + 15V. A seventh terminal 7 of the third operational amplifier OA3 is connected to a resistor R50. A series arrangement of a resistor R33 and a capacitor C4 is coupled between the sixth 6 and the seventh 7 terminal of the third operational amplifier OA3. Not mentioned terminals of the third operational amplifier OA3 are not connected.

The adder circuit 34 comprises a fourth operational amplifier OA4 commercially available as LF353 and the power amplifier 38 comprises an output amplifier OA commercially available as BB3572.

The fourth operational amplifier OA4 has a fourth terminal 4 being coupled to the common line, a fifth terminal 5 being coupled to ground via a resistor R53, a seventh terminal 7 being coupled to a fifth terminal 5 of the output amplifier OA via a resistor R100, an eighth terminal 8 being coupled to the supply voltage +15V, and a sixth terminal 6 being coupled to a junction of a resistor R52, a free end of the resistor R50, and a resistor R51. The free end of the resistor R51 is coupled to the sixth terminal 6 of the second operational amplifier OA 2. The resistor R52 is coupled between the sixth 6 and the seventh 7 terminal of the fourth operational amplifier OA4. Not mentioned terminals of the fourth operational amplifier OA4 are not connected. The output amplifier OA has a first terminal 1 being coupled to the output

O of the controller 22, a second terminal 2 being coupled to the supply voltage + 15 V via a resistor R102, a third terminal 3 being coupled to the supply voltage +15V, a fourth terminal 4 being coupled to ground, a fifth terminal 5 being coupled to a free end of the resistor RIOO, a sixth terminal 6 being coupled to the common line, a seventh terminal 7 being coupled to ground via a capacitor ClOO, and a eighth terminal 8 being coupled to the common line via a resistor R103. A resistor R101, which is arranged in parallel with a series arrangement of a zener diode D106 and a zener diode D107, is coupled between the first 1 and the fifth 5 terminal of the output amplifier OA. A cathode of the zener diode D 106 is connected to a cathode of the zener diode D107. Not mentioned terminals of the output amplifier OA are not connected. The common line is coupled to a suitable negative supply voltage. The circuit components shown in Fig. 4 are as follows:

IC1 ICL8038 - Harris

IC2, IC3 LM741 IC30 LM358

IC50 LF353

ICIOO BB3572

Rl 10 K ohms (variable) R2, R4, R200 1 K ohms

R3, R9, Rll, R13, R32 10 K ohms

R5, R6 4.7 K ohms

R7 500 K ohms R8 1 M ohms

RIO, R12, R14 100 K ohms (variable)

R15 100 K ohms

R31 5 K ohms (variable)

R33 82 K ohms

R34 48.7 K ohms

R50, R52 165 K ohms

R51, R100, R101 20 K ohms

R53 16.2 K ohms

R102, R103 4.5 W

Cl 1 nf

C2 680 pf

C3 50 μf

ClOO 250 pf C200 0.22 μf

C201 0.1 μf

Dl, D20 6.2V ZENER

D30 2V ZENER

DIOO, D101 12V ZENER

Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.